An epoxy resin may be in the form of a liquid or of a solid, depending on its epoxy equivalent weight. A solid epoxy resin may be used for coatings by applying a solution of it to an object and evaporating the solvent or by melting a catalyzed powder on the object. But it is not practical to use a solid epoxy resin as a potting or casting compound since a solvent cannot evaporate from deep within the casting and a melt of the resin would tend to polymerize prematurely.
While liquid epoxy resins would therefore be much more suitable as casting and potting compounds than solid resins, until now it has been difficult to control the flexibility of liquid epoxy resins. Plasticizers have been tried but they are extractable and the resin gradually becomes less and less flexible. Flexible hardeners have also been tried but they are expensive and give only a narrow range of flexibility.
While drying oils are easily made compatible with solid epoxies (see Canadian Pat. 518,956 and Australian patent 205,972), efforts to incorporate drying oils into liquid epoxy resings to flexibilize them have only recently succeeded through the use of a metal chelate catalyst. (See application of Robert H. Runk and Leonard E. Edelman, titled "Liquid Modified Epoxy Resins," filed Sept. 15, 1971 Ser. No. 180,873, now abandoned.)
Ordinarily, the metal chelate catalyst works quite well. But in certain applications it is desirable to prepare a tank car quantity of resin with an anhydride curing agent already mixed in for use over several months to avoid constantly having to mix up small batches. Under these circumstances it was found that the metal chelate catalyst accelerated the cure by the anhydride curing agent so that the resin could not be stored for long periods. This means that another curing agent had to be used or the resin had to be stored as two separate components, and neither solution was very satisfactory.